System and method for lean NOx trap control and diagnosis

ABSTRACT

A system and method for accessing the ability of an emissions control device to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine during each of a series of storage-purge cycles. The device stores the quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry during a storage phase of the cycle. The device releases a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry during a subsequent purge phase of the cycle. The method and system determine, during the purge phase of the cycle, a difference between a predicated time required to purge the device with actual time required to purge the device. The predicted time is computed as a function of a parameter of the device. The parameter varies over time. The method and system modify the parameter used to determine the predicted time during a subsequent one of the series of storage-purge cycles.

TECHNICAL FIELD

[0001] This invention relates to systems and apparatus for accessing the ability of a vehicle emissions control device, such as a lean NOx trap (LNT), to releasably store an exhaust gas constituent.

BACKGROUND OF THE INVENTION

[0002] As is known in the art, the exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O₂). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (λ), engine speed and load, engine temperature, ambient humidity, ignition timing (“spark”), and percentage exhaust gas recirculation (“EGR”). The prior art often maps values for instantaneous engine-generated or “feedgas” constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load (the latter often being inferred, for example, from intake manifold pressure).

[0003] To limit the amount of feedgas constituents from a lean burn engine that are exhausted through the vehicle's tailpipe to the atmosphere as “emissions,” motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx “trap”. Both the upstream and downstream catalyst store NOx when the exhaust gases are “lean” of stoichiometry and releases previously-stored NOx for reduction to harmless gases when the exhaust gases are “rich” of stoichiometry.

[0004] More specifically, in a typical embodiment, the trap stores NOx during lean-burn operation using alkaline metals, such as barium and/or strontium, in the form of a washcoat. The washcoat includes precious metals, such as platinum and palladium, which operate to convert NO to NO₂ for storage in the trap as a nitrate. The NO₂ is stored in the trap in the form of barium nitrate, for example. The trap's washcoat typically also includes ceria, whose affinity for oxygen storage is such that, during initial lean engine operation, a quantity of the excess oxygen flowing through the trap is immediately stored in the trap. The amount of stored oxygen is essentially fixed, although it begins to lessen over time due to such factors as thermal degradation and trap aging.

[0005] The trap's actual capacity to store NOx is finite and the storage efficiency reduces as the trap is being filled-up, hence, in order to maintain low tailpipe NOx emissions when running “lean,” the trap must be periodically cleansed or “purged” of stored NOx. U.S. Pat. No. 5,473,887 teaches the purging of a NOx trap by subjecting the trap to an air-fuel mixture whose air-fuel ratio is rich of stoichiometric, for example, an air-fuel ratio of less than about 13. During the purge event, excess feedgas HC and CO, which are initially consumed in the three-way catalyst to release stored oxygen, ultimately “break through” the three-way catalyst and enter the trap, whereupon the trap's barium nitrate decomposes into NO₂ for subsequent conversion by the trap's precious metals into harmless N₂ and O₂. The oxygen previously stored in the trap is also released during an initial portion of the purge event after the HC and CO break-through the three-way catalyst.

[0006] The time spent in each mode (lean and purge) and the NOx storage/conversion efficiency will not only dictate the tailpipe emissions but also significantly affect fuel economy. Therefore, a proper control strategy which manages the switching between lean and purge mode is crucial to achieve desired emission reduction and fuel economy benefits. The major difficulties involved with controlling an LNT are: 1) The lack of on-board measurements of key variables, such as the feedgas and tailpipe NOx concentration. 2) The changes in the key parameters (such as capacity and efficiency) due to sulfur poisoning and aging.

[0007] Therefore, a need exists for a method and apparatus for accessing the ability of an emissions control device, such as a lean NOx trap, to releasably store an exhaust gas constituent.

SUMMARY

[0008] In accordance with the present invention a system and method are provided for accessing the ability of an emissions control device to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine during each of a series of storage-purge cycles. The device stores the quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry during a storage phase of the cycle. The device releases a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry during a subsequent purge phase, or mode, of the cycle. The method and system determine, during the purge phase, or mode, of the cycle, a difference between a predicated time required to purge the device with actual time required to purge the device. The predicted time is computed as a function of a parameter of the device. The parameter varies over time. The method and system modify the parameter used to determine the predicted time during a subsequent one of the series of storage-purge cycles.

[0009] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0010]FIG. 1 is a schematic of an exemplary system for practicing the invention;

[0011]FIG. 2A is a curve showing the relationship between NOx storage capacity, C, of a new LNT used in the system of FIG. 1 as a function of temperature, T;

[0012]FIG. 2B is a curve showing the relationship between NOx storage efficiency, η_(S), of a LNT as a function of LNT storage level, x, where x=the amount of stored NOx divided by the actual storage capacity, C, of the LNT used in the system of FIG. 1;

[0013]FIG. 2C is a curve showing the relationship between NOx conversion efficiency, η_(C), of a LNT as a function of LNT storage level, x;

[0014]FIG. 2D is a curve showing the relationship between release rate, β, as a function of LNT storage level, x.

[0015]FIG. 3A is a flow diagram of code stored in the system of FIG. 1 used during a LNT storage mode to determine the amount of NOx stored in the LNT during such storage mode and thereby determine when to switch to a purge mode according to one embodiment of the invention;

[0016]FIG. 3B is a flow diagram of code stored in the system of FIG. 1 used during a LNT storage mode to determine a predicted NOx conversion efficiency during such storage mode and thereby determine when to switch to a purge mode according to another embodiment of the invention;

[0017]FIG. 4 is a flow diagram of code stored in the system of FIG. 1 used during a LNT purge mode to determine a predicted purge time of the LNT and the actual purge time of the LNT during such purge mode according to the invention;

[0018]FIG. 5 is a flow diagram of the overall processes in FIG. 3A or 3B and 4 according to the invention;

[0019]FIG. 6 is a flow diagram of the overall process of LNT storage, purge, and parameter adaptation;

[0020]FIG. 7A is a flow diagram of code stored in the system of FIG. 1 used to diagnose the LNT according to one embodiment of the invention;

[0021]FIG. 7B is a flow diagram of code stored in the system of FIG. 1 used to diagnose the LNT according to another embodiment of the invention; and

[0022]FIG. 7C is a flow diagram of code stored in the system of FIG. 1 used to diagnose the LNT according to yet another embodiment of the invention.

[0023] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0024] Referring to FIG. 1, an exemplary control system 10 for a gasoline-powered internal combustion engine 12 of a motor vehicle includes an electronic engine controller 14 having a processor (“CPU”); input/output ports; an electronic storage medium containing processor-executable instructions and calibration values, shown as read-only memory (“ROM”) in this particular example; random-access memory (“RAM”); “keep-alive” memory (“KAM”); and a data bus of any suitable configuration. The controller 14 receives signals from a variety of sensors coupled to the engine 12 and/or the vehicle as described more fully below and, in turn, controls the operation of each of a set of fuel injectors 16, each of which is positioned to inject fuel into a respective cylinder 18 of the engine 12 in precise quantities as determined by the controller 14. The controller 14 similarly controls the individual operation, i.e., timing, of the current directed through each of a set of spark plugs 20 in a known manner.

[0025] The controller 14 also controls an electronic throttle 22 that regulates the mass flow of air into the engine 12. An air mass flow sensor 24, positioned at the air intake to the engine's intake manifold 26, provides a signal MAF representing the air mass flow resulting from positioning of the engine's throttle 22. The air flow signal MAF from the air mass flow sensor 24 is utilized by the controller 14 to calculate an air mass value AM which is indicative of a mass of air flowing per unit time into the engine's induction system.

[0026] A first oxygen sensor 28 coupled to the engine's exhaust manifold detects the stoichiometry of the exhaust gas generated by the engine 12 and transmits a representative output signal to the controller 14, i.e., an ECM (engine control module). The first oxygen sensor 28 provides feedback to the controller 14 for improved control of the air-fuel ratio of the air-fuel mixture supplied to the engine 12, particularly during operation of the engine 12 at or near the stoichiometric air-fuel ratio (λ=1.00). A plurality of other sensors, indicated generally at 30, generate additional signals including an engine speed signal and an engine torque signal in a known manner, for use by the controller 14. It will be understood that the engine torque sensor 30 can be of any suitable configuration, including, by way of example only, an intake manifold pressure sensor, an intake air mass sensor, a throttle position/angle sensor, or an in-line torque sensor.

[0027] An exhaust system 32 receives the exhaust gas generated upon combustion of the air-fuel mixture in each cylinder 18. The exhaust system 32 includes a plurality of emissions control devices, specifically, an upstream three-way catalytic converter (“three-way catalyst 34”) and a downstream lean NOx trap (LNT) 36. The three-way catalyst 34 contains a catalyst material that chemically alters the exhaust gas in a known manner. The LNT 36 alternately stores and converts amounts of engine-generated NOx, based upon such factors, for example, as the intake air-fuel ratio, the trap temperature T (as determined by a suitable trap temperature sensor, not shown or alternatively, estimated based on engine operating parameters), the percentage exhaust gas recirculation, the barometric pressure, the relative humidity of ambient air, the instantaneous trap “fullness,” the current extent of “reversible” sulfurization, and trap aging effects (due, for example, to permanent thermal aging, or to the “deep” diffusion of sulfur into the core of the trap material which cannot subsequently be purged). A second oxygen sensor 38, positioned immediately downstream of the three-way catalyst 34, provides exhaust gas stoichiometry information to the controller 14 in the form of an output signal. The second oxygen sensor's output signal is useful in optimizing the performance of the three-way catalyst 34, and in characterizing the trap's NOx-storage ability in a manner to be described in U.S. Pat. No. 6,308,515 issued Oct. 30, 2001, entitled “Method and Apparatus for Accessing Ability of Lean NOx Trap To Store Exhaust Gas Constituent”, inventors David Karl Bidbner and Gopichandra Surnilla, assigned to the same assignee as the present invention.

[0028] The exhaust system 32 further includes a NOx sensor 40 (or a UEGO sensor in its place) positioned downstream of the trap 36. In the exemplary embodiment, the NOx sensor 40 generates two output signals, specifically, a first output signal that is representative of the instantaneous oxygen concentration of the exhaust gas exiting the vehicle tailpipe 42, and a second output signal representative of the instantaneous NOx concentration in the tailpipe exhaust gas, as taught in U.S. Pat. No. 5,953,907. It will be appreciated that any suitable sensor configuration can be used, including the use of discrete tailpipe exhaust gas sensors, to thereby generate the two desired signals.

[0029] Generally, during vehicle operation, the controller 14 selects a suitable engine operating condition or operating mode characterized by combustion of a “near-stoichiometric” air-fuel mixture, i.e., one whose air-fuel ratio is either maintained substantially at, or alternates generally about, the stoichiometric air-fuel ratio; or of an air-fuel mixture that is either “lean” or “rich” of the near-stoichiometric air-fuel mixture, switching between slightly lean and slightly rich at a frequency of the order of 1 Hz. A selection by the controller 14 of “lean burn” engine operation, signified by the setting of a suitable lean-burn request flag to logical one, means that the controller 14 has determined that conditions are suitable for enabling the system's lean-burn feature, whereupon the engine 12 is operated with a lean air-fuel mixture for the purpose of improving overall vehicle fuel economy. As discussed above, to purge the LNT, the mixture is periodically caused to be rich. The rich purges occur at a rate on the order of 0.01 Hz. The controller 14 bases the selection of a suitable engine operating condition, i.e., stoichiometric or lean burn, on a variety of factors, which may include determined measures representative of instantaneous or average engine speed/engine load, or of the current state or condition of the trap (e.g., the trap's NOx-storage efficiency, the current NOx “fill” level, the current NOx fill level relative to the trap's current NOx-storage capacity, the trap's temperature T, and/or the trap's current level of sulfurization), or of other operating parameters, including but not limited to a desired torque indicator obtained from an accelerator pedal position sensor, the current vehicle tailpipe NOx emissions (determined, for example, from the second output signal generated by the NOx sensor 40), the percent exhaust gas recirculation, the barometric pressure, or the relative humidity of ambient air.

[0030] Thus, the operation of an LNT 36 can be divided into two modes:

[0031] (A) Storage mode (lean engine operation)—the performance of the LNT 36 is characterized by storage efficiency and storage capacity; and

[0032] (B) Purge mode (rich engine operation)—the performance of the LNT 36 is characterized by release rate and NOx conversion efficiency.

[0033] Within these two modes, the LNT 36 is characterized by four key parameters—storage capacity, C, storage efficiency, η_(S), release rate, β, and conversion efficiency, η_(c). These parameters in general are functions of temperature, T, flow rate into the LNT, air-to-fuel ratio, A/F, and feedgas concentration of NOx. FIGS. 2A-2D shows these functions for a typical LNT. More particularly, FIG. 2A shows the functional relationship between the storage capacity of an LNT, (i.e., C) and temperature. The actual storage capacity, C, of the LNT will change with age and sulfur effects. FIG. 2B shows the functional relationship between the storage efficiency, η_(S), of the LNT. It is noted that storage efficiency, η_(S), is a function of x, where x is the amount of NOx stored in the LNT normalized by the actual storage capacity, C, of the LNT. FIG. 2C shows the functional relationship between the LNT conversion efficiency, η_(c) as a function of x. FIG. 2D shows the functional relationship between the release rate, β, of the LNT and x. As noted x is a function of C and thus x will change with age and sulfur effects on the LNT.

[0034] For most applications, the functional relations shown in FIGS. 2A-2D for the storage capacity, C, storage efficiency, η_(S), release rate, β, and conversion efficiency, η_(c) are stored as maps or regressions in the ECM (engine control module) 14. These four parameters (i.e., C, η_(S), β, and η_(c).) together with the dynamic equations (1) and (2) below that govern the behavior of the trap will define the amount of NOx stored in and leaving from the LNT 36 at any time instant. Equations (1) and (2) below represent the LNT 36 system in a mathematical form.

Storage Mode

[0035] $\begin{matrix} {\frac{x}{t} = {\frac{\eta_{s}}{C} \times W_{i\quad n}}} & \left( {1a} \right) \end{matrix}$

 y=W _(in)×(1−η_(s))  (1b)

W _(s) =C×x  (1c)

[0036] where x, as noted above, is the LNT 36 storage level (i.e., x=stored NO_(x)÷Capacity, C), η_(s) is the storage efficiency, C is the storage capacity, W_(in) is the mass flow rate of NO_(x) coming into LNT 36, y is the mass flow rate of NO_(x) leaving LNT 36 and W_(s) is the amount of NO_(x) stored in the LNT 36.

Purge Mode

[0037] $\begin{matrix} {\frac{x}{t} = {{- \beta} \times x}} & \left( {2a} \right) \end{matrix}$

 y=W _(s)×(1−η_(c))  (2b)

[0038] where β is the LNT 36 release rate and η_(c) is the conversion efficiency during purge of the LNT 36.

[0039] The control, adaptation and diagnostics strategy includes four modules (i.e., computer code) stored in the engine controller 14.

Storage Mode

[0040] Module A—This module determines when to switch from the storage mode to the purge mode. This module execute one of two ways processes, i.e., Process A-a or Process A-b, below). It is first noted that the processes use as an initial value for storage capacity, C, the storage capacity determined a priori for a new LNT; i.e., a storage capacity C_(o), as shown in FIG. 2A.

[0041] Process (A-a) (FIG. 3A) Use Stored Amount of NOx to Determine Switching to Purge Mode

[0042] Referring to the flow diagram in FIG. 3A, in Step 300A, this module calculates and monitors the amount of stored in the LNT 36 (i.e., W_(S)) as follows:

[0043] (A) The NOx in-coming flow information, W_(in), is obtained from either: (1) a model (i.e., map) stored in the controller 14 where W_(in) is a function of engine speed, fuel quantity fed to the engine, and EGR rate and ignition time or (2) a NOx measurement obtained from NOx sensor 40′ (FIG. 1) upstream of the LNT 36 when such upstream NOx sensor 40′ is used in place of the downstream sensor 40; and

[0044] (B) Now having W_(in), the system model computes x from Equation (1) using values for C_(o) and η_(S) which assume no aging of sulfur effects, that is assuming an initial new LNT 36. Thus, with the assumed value of C_(o), W_(s) is computed; and

[0045] (C) In Step 302A, when the computed, W_(S), exceeds a predetermined threshold W_(threshold), the process A-a switches to purge mode; otherwise, the process continues and NOx is stored in the LNT 36.

[0046] As will be described, during adaptation mode (FIG. 5), updates are made to the value of the storage capacity C as a result of LNT aging and sulfur effects.

[0047] Process (A-b) Use Predicted Conversion Efficiency to Determine Switching to Purge Mode

[0048] Referring to the flow diagram in FIG. 3B, in Step 300B, this module calculates and monitors, x, (i.e., the stored amount of NOx normalized by the capacity, C) using a model stored in the controller 14 of x as a function of W_(in) where W_(in), is either: (A) a function of engine speed, fuel quantity fed to the engine, EGR rate and ignition time or; (B) a NOx measurement obtained from an upstream NOx sensor 40′ when an upstream sensor 40′ is used in place the downstream sensor 40.

[0049] As shown in FIG. 2C, the conversion efficiency, η_(C), is a function of x. Thus, having, in Step 302B, computed x, the controller 14 determines from FIG. 2C, a predicted LNT conversion efficiency (i.e., η_(c, predicted)). Alternatively, having in Step 302B, computed x, the controller 14 determines from FIG. 2B, the LNT storage efficiency (i.e., η_(S)).

[0050] In Step 304B, if the predicted conversion efficiency (i.e., η_(c, predicted)) is less than a predetermined conversion efficiency (i.e., η_(c, predetermined)), or, if the controller 14 determines from FIG. 2B that the LNT storage efficiency (i.e., η_(s)) is less than a predetermined threshold storage efficiency (i.e., η_(s, threshold)), the process A-b switches to purge mode; otherwise, the process continues and NOx is stored in the LNT 36.

[0051] As will be described, during adaptation mode (FIG. 5) updates are made to the value of the storage capacity C as a result of LNT aging and sulfur effects.

Purge Mode

[0052] Module B—Referring to FIG. 4, module B determines: the purge air-fuel ratio; the time duration of the purge cycle, t_(predicted,) from equation (2); the storage capacity C_(o) and its updated values determined during the adaptation mode (FIG. 5); and the actual purge time duration (i.e., the time when the purge is completed, t_(atual)).

[0053] First, with regard to module B determining the purge air-fuel ratio, it is noted that the purge air-fuel ratio is determined using the previous purge mode information since it is a function of LNT storage level, mass flow rate of exhaust during purge and purge temperature, T. The purge air-fuel ratio is stored in maps or memory, in the controller 14, or calculated by controller 14 in real-time. As noted above, Module A-b includes the calculation for x.

[0054] Next, with regard module B to predict the duration of purge (i.e., the time when the purge is completed, t_(predicted)) in advance using the functional relationship between release rate function, β, and x (i.e., FIG. 2D) stored in the controller 14 and equation (2). The prediction of the time duration of the purge, t_(predicted), is the time t when x becomes 0. As noted above, when using equation (2), the storage capacity C_(o) is used initially and such value is updated during the adaptation mode (FIG. 5).

[0055] To determine when to switch back to the storage mode, Process (B-a) provided by Module B is for normal operation. Process (B-b) provided by Module B is activated only during adaptation, i.e., modification of the capacity C of the LNT 36 because of aging and sulfur effects.

[0056] Process (B-a) Use Model to Predict Time to Switch to Storage Mode (i.e., t_(predicted))

[0057] In Step, 400, the module B predicts the purge time (t_(predicted)) in advance using the release rate function, β (FIG. 2D), and equation (2), i.e., time when x reaches a pre-defined threshold (near to 0). As noted above, when using equation (2), the storage capacity C_(o) is used initially and such value is updated during the adaptation mode (FIG. 5). The predicted purge time, t_(predicted) is forwarded to module C (Step 401) to be described in connection with FIG. 5.

[0058] Process (B-b) Use Sensor 40 to Determine Actual Time Duration of Purge Mode Storage Mode (i.e., t_(actual))

[0059] In Step 500, the controller 14 monitors a sensor at LNT 32 outlet. (As will be described this sensor may be a HEGO sensor, a UEGO sensor, or the NOx sensor 40 shown in FIG. 1). The time at which the purge mode is complete is when the air-fuel ratio reaches a predetermined threshold. The measurement of the air-fuel ratio is provided by the UEGO sensor described above, or by the NOx sensor 40.

[0060] If a HEGO sensor is used, then the controller 14 monitors the voltage level of the HEGO sensor, in Step 502, when the HEGO sensor voltage reaches a pre-defined threshold (V_(HEGO)≧V_(threshold)), a switch is made to the storage mode. (t_(actual)=time when switching).

[0061] If a UEGO sensor is used, then the controller monitors the air-fuel ratio from the UEGO sensor, in Step 504, when, as noted above, the air-fuel ratio reaches a pre-defined threshold (AFR_(UEGO)≦AFR_(threshold)), a switch is made to the storage mode. (t_(actual)=time when switching).

[0062] If the downstream NOx sensor 40 is used, then monitor the air-fuel ratio using the NOx sensor 40, in Step 506, when the air-fuel ratio reaches the predetermined air-fuel ratio a pre-defined threshold AFR_(threshold) (i.e., AFR_(downstream NOX sensor)≦AFR_(threshold)), a switch is made to the storage mode.

[0063] It is noted that the actual time of the purge mode is measured (Step 508). When Step 502, 504, 506, as the case may be, indicated that the LNT has been purged, the purge mode ends. The t_(actual)=time when switching after initiation, or start, of the purge mode is forwarded to Module C to be described in connection with FIG. 5 (Step 509).

Adaptation Mode

[0064] Module C—Module C is used during the adaptation phase. That is, periodically, as preset by the controller 14, the system goes through an adaptation phase. This adaptation phase is used to update the estimate of the LNT capacity, C, from the initial capacity, C_(o,) or from the last undated capacity, C^(original) to a new capacity C^(new). Once updated, the value C in equations (1) and (2) above, are replaced with C^(new), and the updated four functions defined in FIGS. 2A-2D are used subsequently.

[0065] When this module is activated (STEP 600) by engine controller 14, Module B is activated. Thus, the predicted purge time (t_(predicted)) and the actual purge time (t_(actual)) are forwarded to Module C by module B, as described above in connection with FIG. 4 in connection with Steps 401 and 509).

[0066] In Step 602, the module compares the actual purge time tactual and the predicted purge time t_(predicted) (both from Module B).

[0067] In Step 604, when the difference, e, between the actual purge time t_(actual) and the predicted purge time t_(predicted) is less than a prescribed threshold, adaptation is not necessary and the process exit from module. However, if the difference, e, is greater than a prescribed threshold, e_(threshold), adaptation is activated.

[0068] When adaptation is activated (Steps 608 and 610), the capacity of the LNT will be adjusted to eliminate the difference for future operation. The equation (3) describes the adaptation rule in mathematical from.

C ^(new) =C ^(original)×θ^(new)  (3a)

θ^(new)=θ^(old) +Ke  (3b)

e=t _(actual) −t _(predicted)  (3c)

[0069] where C^(new) is the capacity after adaptation, C^(original) is the original capacity, θ is a multiplier for adaptation (which is calculated using e and K) and e is the error (time difference) for adaptation. K is an adaptation gain to be pre-determined and stored in the engine controller Steps 600, 608, 610 and 612. That is, the storage capacity, C, function shown in FIG. 2A is scaled by the factor θ.

[0070] The rule to determine the K (Step 608) is as follows.

[0071] The parameter K is a calibration parameter. In normal operation, K has a relatively small value, typically less than 1. After a de-sulfurization (deSOx) operation, however, K has a relatively large value, but still less than 1, since the capacity changes considerably after the deSOx operation.

[0072] Once the capacity is adapted, then the four key functions defined in FIGS. 2A-2D are changed (or adapted) since those 4 functions are defined as a function of capacity (Step 612) in accordance with:

θ^(new)=θ^(old) +Ke  (3b)

where e=t _(actual) −t _(predicted)  (3c)

[0073] and in Step 612, C^(new) is determined in accordance with:

C ^(new) =C ^(original)×θ^(new)  (3a)

[0074] Further, C^(new) and C^(original) are forwarded to a Module D to be described in connection with FIGS. 7A and 7B (Step 613).

[0075] Also, in Step 614, updates are made to the functions C, η_(s), η_(c), and β (FIGS. 2A, 2B. 2C and 2D, respectively) stored in the controller 14 (FIG. 1)

[0076] Thus, it is noted that after adaptation, x becomes x^(new) where x^(new) is now a function of C^(new) in equations 1a-1c, above.

Storage Mode/Purge Mode Summary

[0077] The processes described above are summarized in the flow diagram in FIG. 6. Thus, a system and method are provided for accessing the ability of an emissions control device, here, in this example, an LNT, to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine during each of a series of storage-purge cycles. The device stores the quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry during a storage phase of the cycle Steps 900 and 902 in FIG. 6. As described above in connection with FIGS. 3A and 3B, the end of the storage mode, Step 904, is determined as a function of the device (e.g., LNT) storage capacity, C, which is initially assumed C_(o) and undated during the adaptation mode FIG. 5.

[0078] The device, here LNT, releases a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry during a subsequent purge phase of the cycle, Step 906. The method and system predict (Step 908), during the purge phase of the cycle, the purge time, t_(predicted) based on a model which as a function of the device (e.g., LNT) storage capacity, C, which is initially assumed C_(o) and updated during the adaptation mode FIG. 5. The actual time duration of the purge phase, t_(actual), is determined by sensors as described in FIG. 6 (Steps 910 and 912).

[0079] The adaptation mode is performed based on some measured engine parameter, such as the number of miles or time the engine has run. For example, in the case of a car or truck engine, every 100 miles or tank full of fuel Step (914). During the adaptation mode, Step 916, a difference between a predicated time required to purge the device, t_(predicted), with actual time required to purge the device t_(actual) is calculated, Step 918 obtained from Steps 401 and 509. The predicted time is computed as a function of a parameter of the device, here the storage capacity, C, which is initially assumed C_(o) and updated during the adaptation mode. The parameter, C, varies over time. The method and system modify the parameter, C, used to determine the predicted time during a subsequent one of the series of storage-purge cycles.

Diagnosis of LNT

[0080] Module D—Module D is used to periodically monitor the status of the LNT 32. Three processes used for this purpose are described below in connection with module D-a, D-b and D-c. With module D-a, (FIG. 6A), if the new capacity, C^(new) becomes less than a prescribed threshold, a fault is declared. With module D-b (FIG. 6B), as an alternative, a difference in capacity, i.e. ΔC=C^(new)−C^(original) can be used instead of the capacity. In this case, ΔC exceeds a prescribed threshold, a fault is declared.

[0081] Considering module D-a, if in Step 700, C^(new) is less than or equal to C^(original) the LNT is declared faulty by controller 14 and a suitable malfunction indicator light (MIL) is activated Step 702), otherwise the diagnosis for that period ends.

[0082] Consider alternative module D-b, the controller 14 calculates a difference in capacity, i.e. ΔC=C^(new)−C^(original) in Step 800. If the calculated difference ΔC is greater than a predetermined difference ΔC_(predetermined) (Step 802), LNT is declared faulty by controller 14 and a suitable malfunction indicator light (MIL) (Step 804) is activated, otherwise the diagnosis for that period ends.

[0083] Another alternative module D-c is shown in FIG. 7C. Here, θ is obtained from module 3 at Step 610. The module D-c computes the absolute value of (θ−1), Step 900. The computed value of the absolute value of (θ−1) is compared with a predetermined threshold θ_(TH), Step 602. If the absolute value of (θ−1) is greater than the threshold, θ_(TH), the MIL is activated, Step 404.

[0084] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims. 

What is claimed is:
 1. A method for accessing the ability of an emissions control device to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine during each of a series of storage-purge cycles, wherein the device stores the quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry during a storage phase of the cycle and the device releases a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry during a subsequent purge phase of the cycle, the method comprising: determining, during the purge phase of the cycle, a difference between a predicted time required to purge the device with actual time required to purge the device, such predicted time being computed as a function of a parameter of the device, such parameter varying over time; and modifying the parameter used to determine the predicted time during a subsequent one of the series of storage-purge cycles.
 2. The method recited in claim 1 wherein the determining comprises: calculating the amount of stored in the device, W_(S), comprising: (A) Obtaining NOx in-coming flow information, W_(in), from either: (1) a stored model where W_(in) is a function of engine speed, fuel quantity fed to the engine, and EGR rate and ignition time; or (2) and a NOx measured by a sensor downstream of the device; and (B) Calculating the amount of stored NOx in the device normalized by the storage capacity of the trap, such storage capacity being the parameter which varies over time.
 3. The method recited in claim 2 including switching from the storage phase to the subsequent purge phase when the calculated W_(S), exceeds a predetermined threshold W_(threshold).
 4. The method recited in claim 1 wherein the determining comprises: calculating x, where x is the stored amount of NOx in the trap normalized by the storage capacity of the trap, such storage capacity being the parameter which varies over time by the capacity, such calculating using a stored model of x as a function of W_(in) where W_(in), is either: (A) a function of engine speed, fuel quantity fed to the engine, EGR rate and ignition time or; (B) a NOx measurement obtained from an NOx sensor upstream of the device.
 5. The method recited in claim 4 including: determining from the calculated x, a predicted device conversion efficiency, η_(c, predicted); and. switching from the storage phase to the purge phase when the predicted conversion efficiency, η_(c, predicted), is less than a predetermined conversion efficiency, η_(c, threshold).
 6. The method recited in claim 1 including: predicting the purge phase time (t_(predicted)) as a function of a time when x reaches a pre-defined threshold; using a determination of air-fuel ratio to determine the actual time duration of purge mode storage mode, t_(actual).
 7. The method recited in claim 5 including: predicting the purge phase time (t_(predicted)) as a function of a time when x reaches a pre-defined threshold; using a determination of air-fuel ratio to determine the actual time duration of purge mode storage mode, t_(actual); comparing the actual purge time t_(actual) and the predicted purge time t_(predicted); when the difference, e, between the actual purge time t_(actual) and the predicted purge time t_(predicted) is greater than a prescribed threshold, e threshold, updating the time varying parameter of the device.
 8. The method recited in claim 6 wherein the time varying parameter is the storage capacity, C, of the device, and wherein the updating is in accordance with: C ^(new) =C ^(original)×θ^(new)  (3a) θ^(new)=θ^(old) +Ke  (3b) e=t _(actual) −t _(predicted)  (3c) where C^(new) is the updated storage capacity, C^(original) is the original capacity, θ is a multiplier and K is an adaptation gain.
 9. The method recited in claim 8 including periodically monitoring the status of the device, comprising determining whether the new capacity, C^(new) becomes less than a prescribed threshold.
 10. The method recited in claim 8 including periodically monitoring the status of the device, comprising determining a difference in capacity, ΔC=C^(new)−C^(original) and a predetermined threshold.
 11. The method recited in claim 8 including periodically monitoring the status of the device, comprising comparing the absolute value of (θ−1) with a predetermined threshold, θ_(TH)
 12. A system for accessing the ability of an emissions control device to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine during each of a series of storage-purge cycles, wherein the device stores the quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry during a storage phase of the cycle and the device releases a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry during a subsequent purge phase of the cycle, the system having a processor programmed to: determine, during the purge phase of the cycle, a difference between a predicted time required to purge the device with actual time required to purge the device, such predicted time being computed as a function of a parameter of the device, such parameter varying over time; and modify the parameter used to determine the predicted time during a subsequent one of the series of storage-purge cycles.
 13. The system recited in claim 12 wherein the determining comprises: calculating the amount of stored in the device, W_(S), comprising: (A) Obtaining NOx in-coming flow information, W_(in), from either: (1) a stored model where W_(in) is a function of engine speed, fuel quantity fed to the engine, and EGR rate and ignition time; or (2) and a NOx measured by a sensor downstream of the device; and (B) Calculating the amount of stored NOx in the device normalized by the storage capacity of the trap, such storage capacity being the parameter which varies over time.
 14. The system recited in claim 13 including switching from the storage phase to the subsequent purge phase when the calculated W_(S), exceeds a predetermined threshold W_(threshold).
 15. The system recited in claim 12 wherein the determining comprises: calculating x, where x is the stored amount of NOx in the trap normalized by the storage capacity of the trap, such storage capacity being the parameter which varies over time by the capacity, such calculating using a stored model of x as a function of W_(in) where W_(in), is either: (A) a function of engine speed, fuel quantity fed to the engine, EGR rate and ignition time or; (B) a NOx measurement obtained from an NOx sensor upstream of the device.
 16. The system recited in claim 15 including: determining from the calculated x, a predicted device conversion efficiency, η_(c, predicted); and. switching from the storage phase to the purge phase when the predicted conversion efficiency, η_(c, predicted), is less than a predetermined conversion efficiency, η_(c, threshold).
 17. The system recited in claim 12 including: predicting the purge phase time (t_(predicted)) as a function of a time when x reaches a pre-defined threshold; using a determination of air-fuel ratio determine the actual time duration of purge mode storage mode, t_(actual).
 18. The system recited in claim 17 including: predicting the purge phase time (t_(predicted)) as a function of a time when x reaches a pre-defined threshold; using a determination of air-fuel ratio to determine the actual time duration of purge mode storage mode, t_(actual); comparing the actual purge time t_(actual) and the predicted purge time t_(predicted); when the difference, e, between the actual purge time t_(actual) and the predicted purge time t_(predicted) is greater than a prescribed threshold, e_(threshold), updating the time varying parameter of the device.
 19. The system recited in claim 18 wherein the time varying parameter is the storage capacity, C, of the device, and wherein the updating is in accordance with: C ^(new) =C ^(original)×θ^(new)  (3a) θ^(new)=θ^(old) +Ke  (3b) e=t _(actual) −t _(predicted)  (3c) where C^(new) is the updated storage capacity, C^(original) is the original capacity, θ is a multiplier and K is an adaptation gain.
 20. The system recited in claim 19 including periodically monitoring the status of the device, comprising determining whether the new capacity, C^(new) becomes less than a prescribed threshold.
 21. The system recited in claim 19 including periodically monitoring the status of the device, comprising determining a difference in capacity, ΔC=C^(new)−C^(original) and a predetermined threshold.
 22. The system recited in claim 19 including periodically monitoring the status of the device, comprising comparing the absolute value of (θ−1) with a predetermined threshold, θ_(TH).
 23. A method for determining an amount of NOx, W_(S), stored in a lean NOx trap, comprising: (A) obtaining NOx in-coming flow information to the trap, W_(in), (B) determining the storage capacity, C, of the trap, such storage capacity varying over time; and (C) determining the storage efficiency of the trap θ_(S); (D) calculating the amount of NOx stored in the trap, W_(S), from the obtained W_(in), the determined storage capacity, C, and the determined storage efficiency, η_(S) of the trap.
 24. The method recited in claim 23 wherein the determining comprises: calculating x, where x is the stored amount of NOx in the trap normalized by the storage capacity of the trap, such calculating using a stored model of x as a function of W_(in) where W_(in), is either: (A) a function of engine speed, fuel quantity fed to the engine, EGR rate and ignition time or; (B) a NOx measurement obtained from a NOx sensor upstream of the device.
 25. The method recited in claim 23 including: calculating x, where x is the stored amount of NOx in the trap normalized by the determined storage capacity, C, of the trap, determining from the calculated x, a predicted device conversion efficiency, η_(c, predicted); and. switching from a storage phase for the trap to the purge phase for the trap when the predicted conversion efficiency, η_(c, predicted), is less than a predetermined conversion efficiency, η_(c, threshold).
 26. The method recited in claim 23 wherein the storage capacity, C, is updated is in accordance with: C ^(new) =C ^(original)×θ^(new)  (3a) θ^(new)=θ^(old) +Ke  (3b) e=t _(actual) −t _(predicted)  (3c) where C^(new) is the updated storage capacity, C^(original) is the original capacity, θ is a multiplier and K is an adaptation gain.
 27. The method recited in claim 26 including periodically monitoring the status of the device, comprising determining whether the new capacity, C^(new) becomes less than a prescribed threshold.
 28. The method recited in claim 23 including periodically monitoring the status of the device, comprising determining a difference in capacity, ΔC=C^(new)−C^(original) and a predetermined threshold.
 29. The method recited in claim 23 including periodically monitoring the status of the device, comprising comparing the absolute value of (θ−1) with a predetermined threshold, θ_(TH).
 30. The method recited in claim 23 including switching from the storage phase to the subsequent purge phase when the calculated W_(S), exceeds a predetermined threshold W_(threshold).
 31. The method recited in claim 30 including: predicting the purge phase time (t_(predicted)) when x reaches a pre-defined threshold; using a determination of air-fuel ratio to determine the actual time duration of purge mode mode, t_(actual); comparing the actual purge time t_(actual) and the predicted purge time t_(predicted); when the difference, e, between the actual purge time t_(actual) and the predicted purge time t_(predicted) is greater than a prescribed threshold, e_(threshold), updating the time varying parameter of the device. 